In a cellular mobile communication system using a W-CDMA (Wideband Code Division Multiple Access) method, a control station, a base station and a mobile station (mobile terminal) execute a predetermined control sequence, whereby the mobile station moving over one cell from another cell during communications can continue the communications. In this control sequence, for example, when detecting the over-the-cell movement of the mobile station, each of the control station, the base station and the mobile station switches over a communication route in synchronization.
By the way, there are at least two types of data channels utilized for the communications between the base station and the mobile station. One type of the data channel is a data channel via which the communications can be performed simultaneously in the plurality of cells, and another type is a data channel via which the communications can be performed only in the single cell. In the cellular mobile communication system using the W-CDMA method, a dedicated channel (DCH) is given as the former type of data channel, and a common channel (HS-PDSCH (High Speed Physical Downlink Shared Channel)) in HSDPA (High Speed Downlink Packet Access) communications is given as the latter type of data channel. In the HSDPA communications, the data is transmitted at a high speed by performing time-scheduling via the common channel, and it is therefore impossible to transmit the same data to one mobile station simultaneously from the plurality of cells.
When moving to another cell, the control sequence when moving over the cell is executed corresponding to these types of the data channels. Non-Patent document 1 given below discloses a control method in the former type of dedicated channel when moving over the cell. According to this control method, a way of how the cell is added and deleted is controlled based on a result of measuring a CPICH (Common Pilot Channel) reception quality (Ec/No (main signal level/interference level), sent from each cell and a result of measuring a path loss in the mobile station.
FIG. 12 is a diagram illustrating a concept of the control method when moving over the cell with respect to the dedicated data channel in case that a cell A and a cell B exist and a result of measuring the CPICH reception quality is the basis. In FIG. 12, the CPICH reception quality from the cell A is expressed by Ec/No_A, and the CPICH reception quality from the cell B is expressed by Ec/No_B.
The mobile station moves in a status of performing the communications in the cell A serving as a main cell (a cell offering the best reception quality in the active cells) and, when a difference between Ec/No_A and Ec/No_B becomes equal to or smaller than an additional threshold value (ΔEc/No FOR ADD) (e.g., 3 [dB]), sets the active cells to the cell A and the cell B, wherein a zone where communications with these plural cells are performed is defined as a soft handover (SHO) zone. In this zone, an instruction etc related to transmission power control is output from, e.g., the main cell.
In the SHO zone, the mobile station receives radio waves simultaneously from the cells A and B and synthesizes the radio waves. A shift to the SHO zone is normally made when the mobile station exists in the vicinity of a border between the cells, however, the weak radio waves from each cell can be compensated by this SHO method. Note that in the SHO zone also, the mobile station can, if the cell offering the best Ec/No is switched, change recognition of the main cell, corresponding to that.
The mobile station moves in the status of setting the plurality of cells to the active cells and performing the communications by use of the SHO method, consequently excludes the cell offering a deteriorated Ec/No value becoming equal to or larger than a delete threshold value (ΔEc/No FOR DELL) (e.g., 9 [dB]) from the active cells, and shifts to single-cell communications from plural-cell communications.
On the other hand, in the control method when moving over a cell in the common channel in the latter HSDPA communications, the simultaneous communications with the plurality of cells do not happen, and hence, just when replacing the cell offering the best reception quality Ec/No of CPICH, the communication cell is only switched over (cell change operation). FIG. 13 is a diagram illustrating a concept of the control method when moving to another cell with respect to the common channel in case that the cell A and the cell B exist and a result of measuring the CPICH reception quality is the basis.
According to this method, in the same way as the control of the dedicated channel, the CPICH reception quality Ec/No measured on the side of the mobile station and sent from each cell is reported to the control station, and the control station selectively determines the best cell by comparing the reception qualities of the respective cells with each other and notifies the base station and the mobile station of movement destination cell information and movement timing. This control method has no difference between the case of moving over a base station and a case of moving over a different cell segment from another different divided cell within the same base station.
In the case of performing the HSDPA communications such as this, the dedicated channel might be simultaneously set up as well as setting up the common channel for the HSDPA. This being the case, in the same way as the cell moving control method in the dedicated channel, with respect to also the cell switching via the common channel for the HSDPA, it is a general practice that only the CPICH reception quality transmitted from each cell is set as the basis for the cell switching.
Moreover, Patent document 1 given below discloses a technology by which the base station mainly collects pieces of reception quality information (CQI (Channel Quality Indicator)) etc of the mobile stations, and performs the cell switching to another cell having a different carrier frequency for the mobile station with occurrence of a call loss and occurrence of deterioration in reception quality. Note that the following are other disclosed documents of the conventional art related to the invention of the present application.    [Non-Patent document 1] Technical Specification Group Radio Access Network, “Radio Resource Control (RRC) protocol specification (Release 1999)”, 3rd Generation Partnership Project, 3GPP TS 25.331 V3.21.0, December 2004, Chapter 14.    [Patent document 1] International Publication WO 2004/112419    [Patent document 2] International Publication WO 2004/075589